Clean up PREF_USABILITY_STUDY_MODE

- Refer to a common default value in DebugSettings
- Make PREF_USABILITY_STUDY_MODE independent of the ResearchLogger
- ResearchLogger uses its own preference through ResearchSettings

multi-project commit with Ie0df836c9d779eba484b522666ec357f4e234823

Change-Id: I88547a2f619db6e7364abbbec12f9f76855dd11a